Upgrade from end-of-life Django 1.11 to 2.2

[colekettler]

Django 1.11 was declared end-of-life in April 2020 and will no longer receive bug fixes or security updates.

As described in azavea/docker-django#107, we're dropping support for Django 1.11 from the azavea/docker-django image and will no longer be publishing updates. This means that in addition to there being no updates to Django, there will not be any more security updates to the underlying OS image.

We should prioritize upgrading to Django 2.2 LTS as soon as it is feasible to do so in order to continue applying important bugfixes and security updates to this application.

Connects #867